Various metals, such as aluminum, are anodized to enhance performance properties. Anodizing is an electrochemical (e.g., electrolytic passivation) process that produces and/or increases the thickness of an oxide coating on the surface of a metal part. For example, metals are commonly anodized to increase corrosion resistance and increase wear resistance. An anodized surface also provides better adhesion for paint, primers and glues than does a bare metal surface. Therefore, anodized structures can be found in various industrial applications, such as in the aerospace and automotive industries.
For example, an anodized metal typically includes an oxide layer or coating on a surface of a substrate. Although anodizing produces a very regular and uniform coating, microscopic fissures in the coating can lead to corrosion. Further, the coating is susceptible to chemical dissolution in the presence of high and low pH chemistry, which results in stripping the coating and corrosion of the substrate. For example, the oxide layer is generally columnar, cellular, and porous and the pores may facilitate corrosion of the underlying substrate. Therefore, various techniques have been developed in an attempt to inhibit corrosion of anodized metals by reducing the number of pores (e.g., fissures), inserting more chemically stable compounds into the oxide coating, or both.
As one example, the open pores can be sealed (e.g., through hydro-thermal sealing or precipitating sealing), such as with a hot dichromate solution after anodizing. The addition of the sealing step may significantly improve the corrosion resistance of the anodized metal by reducing porosity and interstitial pathways that allow for corrosive ion exchange between the external surface and the substrate. However, the hot dichromate solution contains hexavalent chromium, which requires special attention to handling and disposal.
Accordingly, those skilled in the art continue with research and development efforts in the field of corrosion inhibition of anodized metals.